Hair can be removed for cosmetic reasons by various methods. One possible method is heating the hair and the hair follicles to a temperature high enough to result in their coagulation.
Light can be used effectively to remove hair. Light energy is absorbed by the pigment of the hair shaft and then transported as heat to the surrounding tissue. The resulting thermal coagulation of proteins in the hair follicle and other hair supporting structures (i.e., buldge, matrix, blood vessels) prevent subsequent hair growth. It is known that blood is coagulated when heated to the temperature of 70.degree. C. Similarly, heating of the epidermis, the hair and the hair follicle to temperatures of the same order of magnitude will also cause their coagulation and will result in permanent removal of the hair.
The hair growth cycle consists of three stages:
(1) Anagen--the growing stage; PA1 (2) Catagen--transverse stage; and PA1 (3) Telogen--the resting stage.
The Catagen stage lasts only for a relatively short period, therefore it will not be taken into account hereinbelow.
Hair can be efficiently removed only during the Anagen stage. During the Telogen stage, the connection of the hair to its follicles is not strong enough to enable performance of the above described hair removal mechanism.
As shown in FIG. 1, to which reference is now made, the Anagen stage can be devided into three periods, according to the characteristics of the hair growth and the efficiency of the hair removal treatment during each period. In the Initial Period (IP) of the Anagen stage, referenced 1, the hair shaft is too fine and small, so that the energy absorbed by the shaft is not high enough to heat and damage the hair supporting structure, such as the follicle, buldge, etc., and the hair removal treatment cannot be performed efficiently.
In the Final Period (FP) of the Anagen stage, referenced 3, the hair follicle can reach the depth of 4 to 7 mm. Large depth of hair follicle complicates the hair removal treatment, due to limitation of light penetration into the tissue.
The Middle Period (MP) of the Anagen stage, referenced 2, is the optimal time for performing an efficient hair removal treatment.
The efficiency of a hair removal treatment during the different periods of the Anagen stage is schematically shown in FIG. 2.
The efficiency of the treatment is not only a function of the hair growth stage, but depends also on other parameters, such as the characteristics of the device used for hair removal. Possible light sources for performing such a treatment can be any kind of non-coherent light, or coherent light such as: Ruby laser, Alexandrite laser or Diode laser.
FIG. 3 is a schematic illustration of the efficiency of a hair removal treatment performed by a light source with penetration depth smaller than the one of the device used in FIG. 2. As can be seen from comparing FIG. 3 to FIG. 2, the period of time when hair is accessible for a successful hair removal treatment is shorter when the light penetration depth is smaller.
Accordingly, higher light energy, combined with deep light penetration, increases the period of time when hair can be successfully removed. As a result of this, a device including a light source with small energy and small penetration depth requires a larger number of sessions in order to complete an efficient hair removal treatment as opposed to a device including a light source with higher energy. The reason of this is that the period of time such a device can be successfully used is shorter, therefore a smaller quantity of hair can be removed during a single session.
As mentioned above, the efficiency of the hair removal treatment is affected by various parameters, such as the device used for performing the treatment, the location of the treatment and the patient's gender.
Therefore, it is desired to be able to plan the hair removal treatment and to predict the results of each session of the treatment, by taking into account the relevant parameters and their influence on the treatment, for the purpose of performing a more efficient hair removal treatment.